This invention relates to woven papermakers' fabrics and especially to forming fabrics, including those known as fourdrinier wires.
In the conventional fourdrinier papermaking process, a water slurry or suspension of cellulosic fibers, known as the paper "stock", is fed onto the top of the upper run of a traveling endless belt or fabric of woven wire and/or synthetic material. The belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium to form a wet paper web. In forming the paper web, the forming belt serves as a filter element to separate the aqueous medium from the cellulosic fibers by providing for the drainage of the aqueous medium through its mesh openings, also known as drainage holes. In the conventional fourdrinier machine, the forming fabric also serves as a drive belt. Accordingly, the machine direction yarns are subjected to considerable tensile stress and, for this reason, are sometimes referred to as the load bearing yarns. Additionally, the cross machine direction yarns on the bottom surface of the forming fabric are subjected to the abrasive forces of the paper machine elements and, for this reason, are often times referred to as the wear resisting yarns.
Such papermakers' fabrics are manufactured in two basic ways to form an endless belt. First, they can be flat woven by a flat weaving process with their ends joined by any one of a number of well known methods to form the endless belt. Alternatively, they can be woven directly in the form of a continuous belt by means of an endless weaving process. In a flat woven papermakers' fabric, the warp yarns extend in the machine direction and the filling yarns extend in the cross machine direction. In a papermakers' fabric having been woven in an endless fashion, the warp yarns extend in the cross machine direction and the filling yarns extend in the machine direction. As used herein, the terms "machine direction" and "cross machine direction" refer respectively to a direction equivalent to the direction of travel of the papermakers' fabric on the papermaking machine and a direction transverse to this direction of travel. Both methods are well known in the art and the term "endless belt" as used herein refers to belts made by either method.
Effective sheet support and minimal wire marking are important goals in papermaking, especially for the belt in the section of the papermaking machine where the wet web is formed. The fibers in the slurry to form the paper are generally of relatively short length. Accordingly, in order to ensure good paper quality, the side of the papermakers' fabric which contacts the paper stock should provide high support for the stock, preferably in the cross machine direction because paper fibers delivered from the headbox to the forming fabric are generally aligned in the machine direction more so than they are aligned in the cross machine direction. Retaining these paper fibers on the top of the forming fabric during the drainage process is more effectively accomplished by providing a permeable structure with a paper contacting surface grid configuration that increases the probability that paper fibers will be supported. Thus the grid spans in both directions should be shorter than the paper fibers so that a high percentage of bridging occurs.
However, if the grid configuration of papermakers' fabric were designed with only fiber retention in mind, such forming fabrics would probably be delicate and lack stability in the machine direction and cross machine direction, leading to a short service life. As noted above, abrasive wear caused by contact with the papermaking machine equipment is a real problem. The side of the papermakers' fabric which contacts the paper machine equipment must be tough and durable. These qualities, however, most often are not compatible with the good drainage and fiber supporting characteristics desired for the sheet side of the papermakers' fabric.
Hence, the ideal papermaking fabric must be fine enough to support and retain a high percentage of the deposited paper fibers, durable enough to withstand wear and give adequate life, strong enough to resist tensile forces to minimize stretching, and open enough to provide drainage and to simplify cleaning. Meeting these multiple criteria generally requires that two layers of fabric be woven at once by utilizing threads of different size and/or count per inch for the sheet making portion and the wear/stretch resisting portion respectively.
In fabrics thus created from two distinct fabrics, the final fabric would have the desirable papermaking qualities on the surface that faces the paper web and the desirable wear resistance properties on the machine contacting surface. In practice, such papermakers' fabrics are produced from two separate fabrics, one having the qualities desired for the paper contacting side and the other with the qualities desired on the machine contacting side and then the two fabrics are stitched together by additional stitching yarns as a single papermakers' fabric. This type fabric is commonly called a triple-layer or TRI-X fabric.
The main problem with so-called triple-layer or TRI-X fabrics wherein the two fabric layers are connected with additional stitching yarns is that an optimum geometry relationship between the two fabric layers is not generally achievable. In practice, the two fabric layers nest together with the bottom surface of the top fabric down in the top surface of the bottom fabric, that is the yarn systems in both directions, machine direction and cross machine direction, in both fabrics, the top fabric and the bottom fabric, are unstacked relative to each other. Therefore, although the drainage holes in the top fabric may be uniform, the individual drainage paths through the composite structure can vary due to the nesting nature of the totally unstacked structure. This unequal or non-uniform drainage path condition can be further aggravated through the addition of the independent stitching yarns required to tie both fabrics together.
Other undesirable aspects of independently stitched and totally unstacked or intimately nested so-called triple-layer forming fabrics include reduction in potential permeability and susceptibility to stitch yarn failure and subsequent ply separation. The lessened permeability can adversely affect slurry drainage, sheet knockoff capability and fabric cleaning efficiency. The stitching yarn failure can occur externally, that is on the sheet side surface or on the machine side surface, or internally, that is within or between the top fabric and the bottom fabric, depending upon the degree of burial below the respective surfaces in the one case and the amount of movement between the two fabrics in the other case. For obvious geometric reasons the stitching yarn in an independently stitched triple-layer fabric must be a relatively small diameter yarn; hence it is often hard pressed to withstand the imposed tensile and abrasive forces. Yet another drawback of independently stitched so-called triple-layer forming fabrics is increased production costs. Where the stitching yarns are inserted as picks or shutes the weaving time is at a minimum increased in direct proportion to the number of additional strands per inch required to achieve a satisfactory, from the marking standpoint and the structural standpoint, stitching pattern. In the case of a flat woven product (which essentially all triple-layer products have been to date), the subsequent cost of joining needed to make the product endless for operation on the papermaking machine is also increased.
To date no known fabric has incorporated at one time all the qualities, that is maximum fiber support, uniform drainage paths, high permeability, good stretch resistance, and long life potential desirable, for the production of superior paper. It has long been desired to devise such a product for an economical cost that falls within the criteria established in the brown paper market. Since brown paper must be produced at a relatively low cost as compared to other papers, such a fabric would be ideal, and cost effective, for all types of paper.
Accordingly, it is an object of the present invention to provide a papermakers' forming fabric suitable for, but not restricted to, the formation of brown paper.
It is another object of the present invention to provide a papermakers' forming fabric having a papermaking surface with a high fiber support for effective forming and efficient release of the paper web.
Another object of the present invention is to provide a papermakers' forming fabric having uniform drainage paths through the structure from the sheet side surface to the machine side surface.
A further object of the present invention is to provide a papermakers' forming fabric with high permeability and high stretch resistance for effective draining and efficient cleaning with trouble-free running.
Another object of the present invention is to provide such a papermakers' forming fabric while maintaining a durable wear resistant machine element contacting surface.
Still another object of the present invention is to provide a papermakers' forming fabric, the economics of which fall well within that of even brown paper parameters.